Circuit-arrangement for silent tuning in frequency-modulation receivers



Sept. 27, 1955 B. G. DAMMERS CIRCUIT-ARRANGEMENT FOR SILENT TUNING INFREQUENCY-MODULATION RECEIVERS Filed July 19-, 1951 INVENTOR BernhardusGerhardus Dommers By %W AGENT United States Patent CIRCUIT-ARRANGEMENTFOR SILENT TUNING IN FREQUENCY-MODULATION RECEIVERS Bernhardus GerhardusDammers, Eindhoven, Netherlands, assignor to Hartford National Bank andTrust Company, Hartford, Conn., as trustee Application July 19, 1951,Serial No. 237,553 Claims priority, application Netherlands July 28,1950 3 Claims. (Cl. 250-20) The invention relates to acircuit-arrangement for silent tuning in a frequency-modulationreceiver, comprising a frequency demodulator to demodulate the incomingsignal, a source of cut-oft voltage to cut off the frequency demodulatorif the incoming signal remains below the required level, and a rectifiercoupled with an input circuit of the frequency demodulator to produce acontrolvoltage for the release of the frequency demodulator if thesignal exceeds this level.

Use is particularly made of a mixing detector, to operate as thefrequency demodulator.

The term mixing detector is to be understood to mean a mixing tubehaving two control grids and combined with two coupled circuits tuned tothe central frequency of the incoming signal, to which the oscillationsto be demodulated are supplied and each circuit of which is respectivelyconnected to one of the two control-grids. A low-frequency oscillationwhich is, on a first approxi mation, proportional to the frequency ofthe incoming signal, is obtained from the anode circuit of the mixingtube. A preferred embodiment of a mixing detector is that in whichsignal voltages of such high values are supplied to the control-grids ofthe mixing tube that the tube has a saturation characteristic curve forthe signal voltages and, hence, operates not only as a demodulator butalso as a limiting element.

In order to produce the control-voltage to release the receiver, it isadvantageous to have the said rectifier preceded by a circuit tuned tothe central frequency of the incoming signal and having a high circuitquality to which the signal is supplied so that a small detuning of thereceiver entails a great variation of the controlvoltage. The impedancesof the usually slightly overcritical coupled circuits of the mixingdetector then have too small a variation with frequency for the voltagesacross these circuits to furnish, upon rectification, a sufficientlysensitive control-voltage. Use is therefore made of a separate circuithaving a high circuit quality.

If, in general, a third circuit having a high circuit quality is coupledundercritically with one of the two input circuits of the mixingdetector, it has been found that the impedances of these input circuitsas a function of frequency become asymmetrical, that is to say, that anappreciable second harmonic distortion occurs across the output of themixing detector. This is due to the fact that coupling of either of thetwo input circuits with the said third circuit is attended with couplingof the other circuit with this third circuit, which results in the saidasymmetry.

The distortion of the demodulation characteristic curve is avoided inthat, according to the invention, either of the two input circuits ofthe frequency demodulator, more particularly a mixing detector, isconnected to a point of constant potential through a comparatively largecapacitor which, together with an inductance and a second capacitor,form a resonator circuit having high circuit quality, the voltage ofwhich is fed to the said rectifier to produce the said release voltage.

2,719,221 Patented Sept. 27, 1955 In order that the invention may bemore clearly under- I stood and readily carried into effect, oneembodiment will drawing.

Referring to the single figure of the drawing, 1 designates amixingtube; a pre-amplification tube; 3 and 5 two coupled circuits tunedto the central frequency of a frequency-modulated incoming signal,connected to the control grids 4 and 6, respectively, of the mixingtube 1. An incoming signal is supplied amplification in the tube 2 tothe circuit 3, which isinductively coupled with the anode circuit of thetube 2, and to the circuit 5, which is coupled with the circuit 3,respectively, the phase shift between the voltages across the circuits 3and- 5, respectively, varying with the frequency of the incoming signal.The tube 1 combined with the circuits3and 5 thus operates as a mixingdetector for the incoming signal and, particularly if the signal across.the circuits 3 and 5 is sufliciently high, it also opcrates as alimiting element.

If the voltage across the circuits 3 and 5 is excessively low, eitherdue to false tuning or due to the absence of a transmitter, theremainder of the receiving channel is suppressed with the use of acut-off voltage source 8 included in the circuit of the firstcontrol-grid 7 of the mixing tube 1. The cut-off is released at asuflicient strength of the incoming signal by rectifying this signalwith the use of a rectifier 9 and by supplying it, subsequent toamplification in the tube 18, to the control-grid 7 of the tube 1 with apolarity opposite to that of the cutoff voltage source 8.

A frequency-modulation receiving circuit-arrangement having silenttuning with the use of a cut-off voltage source is known per se (Frenchpatent specification No. 925,979), in which with the two circuits of afrequency demodulator is coupled a third circuit having a high circuitquality, the rectified output voltage of which releases the cut-offproduced by this voltage source, when the incoming signal has reachedthe required value. However, it is found that in this case invariably anasymmetrical demodulation characteristic curve occurs, which, inaccordance with the basic idea of the invention, is to be attributed tothe fact that an inductive coupling of a third circuit having a highcircuit quality with one of the two input circuits of the demodulator isinvariably attended with a coupling with the other circuit. Thisasymmetrical demodulation characteristic curve results in secondharmonic distortion.

According to the invention capacitative feed coupling is used and thebottom side of the inductance 11 of the circuit 3 is connected through acomparatively large capacitor 12 of, for example, 3000 pfs., to a pointof constant potential, for example, ground, whereas this capacitor 12,together with an inductance 15 and a second capacitor 16, constitutes acircuit having a high circuit quality. The voltage across this circuit,more particularly the voltage across the second capacitor 16, issupplied to the said rectifier 9, after which the rectified outputvoltage is supplied through a smoothing filter 17 to the grid of atuning indicator tube 18, the anode resistor 19 of which has producedacross it an amplified voltage which, subsequent to voltage divisionthrough resistors 20 and 21, releases the cut-off produced by the source8 when the incoming signal is of suflicient strength.

So that a strong transmitter, upon reception, even in the event of falsetuning, will not produce a low-frequency signal across the anode circuitof the tube 1, provision is made of an auxiliary rectifier 24 comprisinga threshold voltage source 22 which becomes conductive when the voltageacross the circuit 3 exceeds some predetermined value and supplies,subsequent to filtering subsequent to with the use of a filter 23, avoltage to the grid of tube 2 which counteracts a further increase ofthe, incoming.

signals beyond this maximum value.

What I claim is:

1. In a frequency modulation receiver, a silent tuning device responsiveto an applied frequency modulated signal, said tuning device comprisinga frequency demodulator for demodulating said signal, said demodulatorhaving first and second resonant input circuits therefor, means tosupply a cut-oil voltage to said demodulator having a magnitude at whichsaid demodulator in the absence of a signal above a predetermined levelis normally inoperative, a tuned network of relatively high circuitquality with respect to the circuit quality of said first and secondresonant input circuits and resonant to the central frequency of saidfrequency modulated signal, said network including first and secondcapacitances and an inductance, said first capacitance having arelatively high value and being coupled between one of said demodulatorinput circuits and a point of constant po tential, means including arectifier coupled to said network to develop a control voltage dependenton the level of the applied signal, and means to supply said controlvoltage to said demodulator to counteract said cutoff voltage to anextent releasing the demodulator in the condition wherein said signalexceeds said predetermined level.

2. A tuning device as set forth in claim 1 wherein said indicatorelectron ray tube having a control grid coupled ,.to.said rectifier, thecontrol voltage. being obtained. from the output of said tube.

3. In a frequency modulation receiver, a silent tuning device responsiveto an applied frequency modulated signal, said tuning device comprisinga frequency demodulator for demodulating said signal, means to supply acut-off voltage to said demodulator having a magnitude at which saiddemodulator in the absence of a signal above a predetermined level isnormally inoperative, a tuned network and resonant to the centralfrequency of said signal including first and second capacitances and aninductance, said first capacitance being coupled between an input ofsaid demodulator and a point of constant potential, means including arectifier coupled to said network to develop a control voltage dependenton the level of the applied signal, and means to supply said controlvoltage to said demodulator to counteract said cut-off voltage andrelease the demodulator in the conmeans to develop a control voltageincludes a tuning dition wherein said signal exceeds said predeterminedlevel.

References Cited in the file of this patent UNITED STATES PATENTS2,197,516 Case Apr. 16, 1940 2,353,468 Holst et al. July 11, 19442,400,948 Peterson May 28, 1946 2,455,450 Thompson Dec. 7, 1948

